With the rapid development of nanotechnology, magnetic nanoparticles are currently being widely studied. It has long been known that the physico-chemical properties of magnetic nanoparticles can be vastly different from those of the corresponding bulk material (Selvan, S. T. et al., 2002, Phys. Chem. B, 106, 10157-10162).
As a result, the magnetic nanoparticles will display superparamagnetism, which means that, depending on temperature and dimensions, the particles are attracted by a magnetic field, but retain no residual magnetism after the field is removed (Stavroyiannis, S. et al., 1998, Appl. Phys. Lett., 73, 3453-3458). Therefore, suspended superparamagnetic particles can be removed from the suspension by an external magnet, but they do not agglomerate (i.e. they stay suspended) after removal of the external magnetic field. These nanoparticular materials often exhibit very interesting electrical, optical, magnetic, and chemical properties, which cannot be achieved by their bulk counterparts (Ashoori, R. C., 1996, Nature, 379, 413-419).
Magnetic iron oxide nanoparticles were proposed as magnetic pigments in recording and magnetic storage media, catalysis, magnetic fluids, magneto-optical devices, controlled drug delivery, image intensifying agents for nuclear magnetic resonance imaging and magnetic induced cancer therapy (Machala, L. et al., 2007, J. Phys. Chem. B, 111, 4003-4018). The perceived advantage of using particles of nanometric size instead of micro- or sub-micrometric size is their larger surface area for the attachment of the enzymes, enabling the preparation of nanostructured biomaterial with a high bio-element loading per mass unit. The others advantages consist of the simple and fast possibility to immobilize bioelements, that can be implemented just before doing a biosensing experiment, and the straightforward adjustment of the amount of antibody magnetically deposited on the electrode surface.
The development of uniform nanometer sized particles has been intensively pursued because of their technological and fundamental scientific importance. There have been various methods developed for the preparation of paramagnetic nanoparticles (Mornet, S. et al., 2004, J. Mater. Chem., 14, 2161-2175; Laurent, S. et al. 2008, Chem Rev., 108, 2064-2110). Among the chemical methods reported in literature and currently used to synthesize magnetic nanoparticles for medical applications, the following can be found: microemulsion technology, sok gel syntheses, sonochemical reactions, hydrothermal reactions, hydrolysis and thermolysis of iron complex precursors, flow injection syntheses and electrospray syntheses (Laurent, S. et al. 2008, Chem Rev., 108, 2064-2110).
The most commonly used protocol involves co-precipitation of ferrous and ferric ions in basic solutions. In most cases, in order to prevent particle aggregation during synthesis, to optimize dimension homogeneity, and to permit bioelement immobilization, a water-in-oil reverse micelle suspension is used, with the aid of a surfactant molecule (Capek, I., 2004, Adv. Coll. Interf. Sci., 110, 49-74). Polymers such as dextran, polyvinyl alcohol, and diethyl-aminoethyl-starch were added to coat the particles for better stability, before or after the formation of iron oxide particles (Lee, J. et al., 1996, J. Coll. Interf. Sci., 177, 490-494; Bergemann, C. et al., 1999, J. Magn. Magn. Mater. 194, 45-52).
Furthermore, particles such as crosslinked iron oxide (CLIO) (Wunderbaldinger, P. et al., 2002, Acad. Radiol., 9, 5304-5306; Schellenberger, E. A. et al., 2002, Mol. Imaging, 2, 102-107), ultrasmall superparamagnetic iron oxide (USPIO) (Kooi, M. E. et al., 2003, Circulation, 107, 2453-2458; Keller, T. M. et al., 2004, Eur. Radiol., 14, 937-944), and monocrystalline iron oxide nanoparticles (MIONs) (Funovics, M. A. et al., 2004, Magn. Reson. Imaging, 22, 843-850; Krause, M. E. et al., 2004, Magn. Reson. Imaging, 22, 779-787) had all been developed as imaging agents in magnetic resonance imaging (MRI). Depending on surface functionalities introduced on particle surface, some of the particles are likely to be taken up by macrophages and immune cells, and can be used to image lymph nodes and inflammatory tissues. In other studies, specific ligands were attached on the particle surface, in order to image the localization pattern of the target molecules (Schellenberger, E. A. et al., 2002, Mol. Imaging, 2, 102-107).
Otherwise, magnetic nanoparticles can be coated with silica and the hydrolyzed silica surface contains a high coverage of silanol groups, which can easily be anchored with defined and generic surface chemistries (Laurent, S. et al. 2008, ref. cit.).
However, the synthesis of superparamagnetic nanoparticles is a complex process because of the colloidal nature of their dispersions. The first main chemical challenge consists of finding out experimental conditions, leading to a monodisperse population of magnetic grains of suitable size. The second critical point is to identify a reproducible process that can be industrialized without any complex purification procedure, such as ultracentrifugation, size-exclusion chromatography, magnetic filtration, or flow field gradient. Furthermore, nanoparticles should not aggregate in water and should form stable suspensions in water.